Testing of soil samples generally is necessary when contemplating the design and construction of buildings and similar structures. Specifically, the designer needs to know the type, nature and characteristics of the sub-surface materials so that a suitable foundation can be constructed to support the structure.
Evaluation of soil generally involves a two step procedure. The first step, known as penetration testing, requires that a rod, or the like, be driven a predetermined depth into the tested soil. The amount of work required to drive the rod reveals the load bearing characteristics of the soil. The second step of the evaluation procedure involves extraction of a sample of the soil for off-site testing.
Generally, these two procedural steps can be accomplished simultaneously. In a process known as "drive sampling", a sampler spoon is driven into the soil at the bottom of the bore hole. Specifically, once the bore hole has been completed to the proper elevation for sampling, and has been properly cleaned of loose soil and debris, a sampler spoon--such as a split tube sampler--is screwed to an assembly of drill rods. This assembly is lowered, spoon first, down into the bore hole. A drive head or anvil is attached to the top of the drill rods which extend out of the bore hole. A drive hammer, of a preselected weight, is then dropped repeatedly through a fixed elevation, onto the drive head. The distance through which the sample spoon is driven, and the number of blows applied to the drive head (the blowcount) are recorded for further analysis to determine the physical resistance of the soil. Throughout the driving process, soil is introduced into a cavity in the sampler spoon through an opening in the bottom thereof. Upon extraction of the drill rods and sample spoon from the bore hole, the soil sample can be removed from the cavity for further analysis.
Several inherent problems exist in the current test apparatus. To begin with, it is necessary to undergo the time consuming process of connecting and disconnecting successive drill rods, each typically 5 feet or 10 feet in length, every time the sample spoon is inserted into and retrieved from the bore hole. Furthermore, each drill rod, itself, is costly and adds considerable weight to the overall assembly necessitating a relatively large winch to handle positioning of the assembly.
In addition to the cost and weight disadvantage, the accuracy of the test results of the current system is not very good. Indeed, according to the American Society for Testing and Materials Standard D 1586-84 "Standard Method for Penetration Test and Split-Barrel Sampling of Soils", variations of 100% or more in the blowcount representation of the penetration resistance of the soil have been observed when using different apparatus and drillers for adjacent test boring in the same soil formation. With care, a driller may reduce the variation in the blowcount to about 10%.
These dramatic errors can be attributed, in part, to a dirty, rusty or defective cathead--the rotating drum or windlass around which the operator wraps a rope to lift and drop the hammer by successively tightening and loosening the rope turns around the drum; old and/or oily cathead rope; and/or oversized or defective rope sheaves. Additionally, errors can be attributed to kinetic energy absorbed or disposed by long lengths of connected drill rods and/or by cracked or worn joints between successive drill rods. It is difficult to calculate the amount of kinetic energy loss and derive a common correction factor to mathematically account for this deficiency; because the length of the connected drill rods vary as the boring is advanced and condition of the rods vary from boring to boring. Operator error may further amplify the inherent errors of the system, as it is necessary for the operator to lift the hammer an estimated 30 inches successively and accurately many times throughout the average test cycle.
Despite the continual need for accurate soil testing, most soil testing systems are incapable of providing accurate and reliable blowcount penetration tests, conducted at the bottom of bore holes.